Bring a large pot of generously salted water to a rolling boil. Add pasta and cook according to package directions until al dente. Reserve 1/2 cup pasta water before draining.

While pasta cooks, pat chicken cubes dry with paper towels. Season generously with salt, pepper, and half the Italian seasoning.

Melt 1 tablespoon butter in a 12-inch or larger skillet over medium-high heat. Add chicken in a single layer and sear for 3-4 minutes per side until golden brown and cooked through to 165 degrees F. Remove chicken to a plate and set aside.

Reduce heat to medium and melt remaining 2 tablespoons butter in the same skillet. Add minced garlic and red pepper flakes if using. Cook for 30-60 seconds, stirring constantly, until fragrant.

Pour in chicken broth to deglaze the pan, scraping up browned bits from the bottom with a wooden spoon.

Stir in heavy cream and remaining Italian seasoning. Bring to a gentle simmer and cook for 2-3 minutes to thicken slightly.

Reduce heat to low. Gradually sprinkle in freshly grated Parmesan cheese, stirring gently until completely melted and sauce is velvety smooth.

Return chicken and any accumulated juices to the skillet. Add drained pasta. Using tongs, gently toss everything together until pasta and chicken are thoroughly coated in sauce.

If sauce is too thick, stir in splashes of reserved pasta water until desired consistency is reached. Taste and adjust salt and pepper if needed. Serve immediately, garnished with fresh parsley.